Hi,
I used these formulae to code a loan calculator with reverse calculating the three variables below,
Payment, Amount borrowed, and Number of months
Unfortunately, I have no idea how to get at the interest rate.
I would be a happy beginner if I could achieve this ultimate step.
Thanks for your help.
A : Amount borrowed
N : Number of months (in this example)
I : Interest rate = I / 1200 (convert annual interest rate to monthly rate)
P : Payment
The payment is calculated by this formula:

P = (A x I)/(1-(1 + I)^(-N))

The amount borrowed is given by the formula:

A= P x (1-(1 + I)^(-N)) / I

And the number of months is given by the formula:

N = (log(P) - log(P - A x I))/(log(1 + I) with (P - A x I) > 0